1. Field of the Invention
The present invention relates to a system and method for moving an object using piezo actuators and, more particularly, to a system for moving an object associated with lithographic apparatus.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist).
In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
In a lithographic apparatus, a system for moving an object using piezo actuators may be employed to move objects, such as a lens element or a mirror used for controlling a position and angle of an exposure laser beam. A “piezo” material is a known material that deforms in response to a voltage or charge. Different types of piezo materials exist. For example, one type of piezo material changes in length while other types shear in response to an applied control signal. Hereinafter, piezo materials that either increase or a decrease in length will be referred to as a “lengthening piezos”, while piezo materials that shear will be referred as a “shearing piezos.”
The change in length or the shear distance is substantially linear relative to a charge supplied by the applied control signal, if no external load is present. An object attached or connected to the piezo actuator, i.e. an external load, moves with the piezo material. The lengthening or shearing of the piezo material is limited. Therefore, an object may only be moved over a relatively small distance when using a linear mode of the piezo actuator.
To increase the range for moving an object, it is known to combine the above-mentioned piezo types in order to create piezo actuator that can both lengthen and shear. With a number of such piezo actuators and a suitable sequence of lengthening and shearing, an object may be moved in a certain direction over a greater distance.
For example, using a number of piezo actuators adapted to lengthen and shear, it is possible to engage an object. Then, in a first step of the sequence, a first actuator releases the object by contracting away from the object and, in a second step, the first actuator shears to the left, while the second actuator, still engaging the object, shears to the right taking the object along with it. In a third step, the first actuator engages the object again by expanding, while remaining in its sheared state.
A second actuator releases its engagement with the object by contracting, while remaining in its sheared state. Next, in a fourth step, the first actuator shears to the right, back to its original state taking the object along with it. The second actuator also shears back to its original state. In a fifth step, the second actuator engages the object again by expanding. After the fifth step, both piezo actuators are back in their original relaxed state. The object, however, has been moved twice to the right and has thus moved twice a predefined, possibly maximum, shear distance of a single piezo actuator. The sequence may be repeated any number of times until the object reaches its target position. Hereinafter, the sequence will be referred to as a stepping sequence or stepping mode.
It is a disadvantage of the stepping mode that even for small moves, the piezo actuators need to contract and expand. The lifetime of the piezo actuators depends on the number of contractions or shears. Thus, the lifetime of the piezo actuators is reduced by contracting and expanding for small moves whereas only shearing could suffice. However, as mentioned above, the range for moving an object in a linear mode is very small.